1. Field of the Invention
The present invention relates to techniques for optically reproducing information.
2. Description of the Related Art
FIG. 2 shows one exemplary configuration of a magnetooptical recording/reproduction device which is one of prior known optical recording/reproduction devices. Laser light emitted from a laser 311 mounted on an optical head 3 is collimated by a collimator lens 312 into parallel rays of light, which are guided via a beam splitter 324 to a lens 321 that forms a light spot 21 on a magnetooptical recording medium 8. The position of the light spot 21 on the magnetooptical recording medium 8 is controllable by movement of the lens 321 and the optical head 3 under control of a light spot scan control means 63. Reflection light from the magnetooptical recording medium 8 is guided by the beam splitter 324 toward a photodetection means 33. A reproduction signal from the photodetection means 33 is processed by a reproduction circuit 93 for conversion to reproduction data. These overall reproduction operations are under control of a controller 55.
As a method for reproducing information recorded at a high density using the optical recording/reproduction device, a magnetic super-resolution reproduction method is proposed in, for example, Published Unexamined Japanese Patent Application Nos. 3-93058 and 3-93056, which method utilizes a temperature increase within a light spot during reproduction to reproduce information corresponding to such temperature increase part, or to reproduce information of those portions other than the temperature increased part in the light spot.
In this case, certain light of substantially constant intensity is continuously irradiated as the reproduction light. Alternatively, as disclosed in JP-A-56-37834, pulsed light is irradiated at extra high frequencies. With such an arrangement, however, the pulsed light irradiation is effectively equivalent to continuous light irradiation because of the fact that the repeat frequency of such pulses is as high as several hundreds of megahertz or greater so that both the temperature on the optical recording medium and a reproduction output obtainable from reflection light are hardly responsive to a reproduction output of the pulsed light.
The above-described prior art has a problem of an inability to reproduce any high-density recorded information because of a decrease in effective reproduction signal quality due to the fact that only part of the information of a light spot contributes to reproduction during reproduction of high-density recorded information, thereby reducing the resultant reproduction signal output.
It is therefore an object of the present invention to provide a technique capable of avoiding the problem and of reproducing high-density recorded information at excellently high output with enhanced quality.
To attain the foregoing object, the present invention employs specific means as will be set forth below.
When an optical recording medium is irradiated with light for reproduction of information on the optical recording medium by use of reflection light of the light, light is irradiated intermittently, or alternatively in the form of pulses, and reflection light is detected at at least two time points during irradiation of such intermittent light, thus obtaining a reproduction signal through mutual processing of resultant detection signals.
This makes it possible to detect only a change in reflection light during irradiation of the light, which in turn makes it possible to obtain a high signal output while enabling suppression of those components other than such a change component, thus obtaining a reproduction signal of excellent quality.
Additionally, it is also preferable that reproduction is done while causing the optical recording medium to reversibly change or vary in optical nature by irradiation of intermittent light.
It is thus possible to read, as a signal, only a change component of reflected detection light only at a specific part of the optical recording medium where the optical nature has been altered due to irradiation of intermittent light, which in turn leads to achievement of reproducibility with increased resolution.
Preferably, the reversible change in optical nature of the optical recording medium makes use of a reversible change in optical nature caused by a change in temperature of the optical recording medium due to the intermittent light.
With such an arrangement, a significant change of reflected detection light is obtainable before and after irradiation of the intermittent light, thus obtaining an increased signal output.
It is also preferable that a series of intermittent light pulses may be comprised of at least two light pulses.
The stability of reproduction light is thus improved, obtaining good reproduction signal quality.
The invention provides an optical recording/reproduction device including light irradiation means for irradiating light to an optical recording medium, photodetection means for detecting reflection light of the light, light modulation means for recurrently irradiating light intermittently, first synchronous detection means for detecting an output from the photodetection means in a way synchronized with the intermittent light, second synchronous detection means for detecting an output from the photodetection means at a time point different by a fixed time duration from that of the first synchronous detection means, and processor means for performing operational processing of the outputs of the first synchronous detection means and the second synchronous detection means.
A detection time difference between the first synchronous detection means and the second synchronous detection means may be shorter than a time period of irradiation of the intermittent light.
With these arrangements, it becomes possible to detect only a change component of the reflection light during irradiation of the same intermittent light, thus obtaining a high signal output while enabling suppression of those components other than such a change component, which leads to an ability to obtain a reproduction signal of excellent quality.
The invention provides an optical recording/reproduction device including light irradiation means for irradiating light to an optical recording medium, photodetection means for detecting reflection light of the light, light modulation means for recurrently irradiating light intermittently, delay means for delaying an output from the photodetection means for a predetermined time period, processor means for processing the output delayed by the delay means and the output from the photodetection means, and synchronous detection means for detecting an output from the processor means in a way synchronized with the intermittent light.
The delay time of the delay means may be less than an irradiation time period of the intermittent light.
With these arrangements, it is possible to detect only a change component of the reflection light during irradiation of the intermittent light, thereby obtaining a high signal output while enabling suppression of those components other than such a change component, thus obtaining a reproduction signal of excellent quality.
Further, the intermittent light irradiation time may be shorter than the irradiation interval of the intermittent light.
This makes it possible to establish an appropriate cooling time of the optical recording medium to thereby ensure that a temperature change occurs stably, thus reliably obtaining the intended signal.
It is desirable that the intermittent light irradiation time, Tp, satisfies the following relation:
2 nanoseconds less than Tp less than D/y/4
where D is the size of a light spot as formed on the optical recording medium during reproduction, and v is the velocity or speed of the light spot relative to the optical recording medium.
By setting Tp greater than 2 nanoseconds, it becomes possible to reliably provide a temperature rise of the optical recording medium, thus rendering the signal stably obtainable. In addition, by setting the irradiation time at a time taken for the light spot to move a distance less than or equal to xc2xc of the light spot diameter, those components of the reflection light other than the signal components become substantially identical at the instants before and after irradiation of light pulses, thereby making it possible to effectively suppress any unnecessary components.
A differential circuit is usable as the processor means. This makes it possible to detect a signal component alone with unnecessary components suppressed, thus obtaining excellent reproduction signal quality.
An operation of the present invention will be explained with reference to FIG. 5.
FIG. 5(c) shows an example of intermittent light 501 to be irradiated in the present invention. Due to the thermal action of the irradiation light, a temperature distribution 502 on an optical recording medium varies as shown in FIG. 5(b). Here, the medium used herein may be the one as taught by JP-A-3-93056, for example, which changes in optical nature at or above a certain temperature (referred to as a xe2x80x9cmask formation temperaturexe2x80x9d) and renders effectively xe2x80x9cinvisiblexe2x80x9d (or masks) the information recorded on the medium (recording magnetic domains).
FIG. 5(a) is a diagram for comparing the visibility of the information on the optical recording medium at a time point immediately after beginning of light irradiation with that at a time point immediately prior to completion of the light irradiation. In the state 92 just before completion of the light irradiation, a significant change is observable at the center of a light spot between the light irradiation beginning just-after state 91 and the light irradiation completion just-before state 92. This is because a mask is formed at the light spot center due to an increase in temperature. Then, by processing the reproduction signals resulting from these two states, it is possible to detect only a change component between such two states. Here, one example is shown wherein differential processing is done by a differential detector 503.
At this time, since the light spot is slightly moved between the state 91 and the state 92, a change might also be observable in a signal from a low temperature part. However, as long as such movement remains less than or equal to xc2xc the light spot diameter, such a change is not optically resolvable, so that any signal change at a low temperature part will be negligible in practical use.